Disable P2 mirrors by default

Having p2 mirrors enabled has some drawbacks as it can lead to
instabilities see eclipse-equinox/p2#414 also
test can fail because a mirror can't be reached or is slow.

Because of this we should disable mirrors all together and only
selectively enable them for tests where this is required.

tycho-testing-harness/src/main/java/org/eclipse/tycho/test/AbstractTychoIntegrationTest.java

@@ -90,6 +90,9 @@ protected Verifier getVerifier(String test, boolean setTargetPlatform, File user
 
         Verifier verifier = new Verifier(testDir.getAbsolutePath());
         verifier.setForkJvm(isForked());
+        if (isDisableMirrors()) {
+            verifier.setSystemProperty("tycho.disableP2Mirrors", "true");
+        }
         String debug = System.getProperty("tycho.mvnDebug");
         if (debug != null) {
             System.out.println("Preparing to execute Maven in debug mode");
@@ -146,6 +149,14 @@ protected Verifier getVerifier(String test, boolean setTargetPlatform, File user
 
     }
 
+    /**
+     * can be overridden by subclass to explicitly enable mirrors, by default they are disabled.
+     * 
+     */
+    protected boolean isDisableMirrors() {
+        return true;
+    }
+
     protected boolean isForked() {
         return true;
     }